We will merely mention in passing, that the most complex animals, in the various stages of fetal development through which they pass, correspond to the types of structure which are permanent in the lower forms of animal life. Thus, in the zoological chain, there are beings of all grades, from the most simple in structure to the most complex; and the most complex animal, in its development from the ovum or egg, passes through all these grades of structure, ending in that which is above all, and distinctively its own. 'Without going into tedious details, man presents, as regards the most important of his constituent structures, his nervous system, the successive characteristics of an avertebrated animal, a fish, a turtle, a bird, a quadruped, a quadrumanous animal, before he assumes the special human characteristics.' (Draper.)
Our purpose being to show that while complexity of structure is constantly increasing, unitization, or the organized dependence of one part on another, is, at the same time, becoming more complete, we shall refer briefly to the comparative anatomy and physiology of animals. There is in this connection such wealth of material—a long chain of animal beings with all grades of structure from very simple to very complex; each complex animal, in its development from the ovum, passing through all the lower types of structure in succession; so many new organs and functions arising in the course of this development; each organ so arising, becoming, in its turn, more complex in structure, more specialized in function, and more dependent on the office of other organs;—in the midst, I say, of all this wealth of material, indicated here in a great general and imperfect manner, the difficulty, in so brief an exposition as this, is to know what facts to seize upon as calculated to illustrate most aptly the principle under consideration.
The development of the senses, with reference to their organs, nerves, and functions, presents a striking illustration of increasing complexity.
In the lowest forms of animal life, we find general sensibility only, and it is claimed that this exists in the lowest forms, without even the presence of nerves. But as we rise higher in the scale, the special organs of sense gradually become developed—one new sense after another appears; but this is not the only line of increasing complexity. When an organ of sense first appears, its function is of the simplest character; and it is only when we reach the highest types of animal life that it performs the greatest variety of offices peculiar thereto. That of touch is, at first, but crude and simple, becoming delicate and complicated only in the highest types. The sense of pain is a differentiated function, possessed only in a slight degree by reptiles and fishes, and probably not at all by animals still lower in the scale.
The eye-spots of star fishes and jelly fishes simply distinguish light from darkness, much as we do with our eyes closed. There are many degrees of development from this condition of the inferior organism to that of the human eye, which distinguishes the nicest shades of color, distance, form, and size of objects, and the play of passion on the human countenance.
The same variety of function is acquired by the ear in its development from its simplest to its most complex form. In the higher animals, the organ of hearing is formed of three parts, an external, middle, and internal portion; but in birds the external ear is wanting; in fishes both the external and middle parts are wanting; in mollusks it is reduced to a simple sack of microscopic dimensions, filled with a liquid in which there are otolithes, or pebbly substances. Such an organ can distinguish noises only; it can recognize nothing of the infinite variety of articulations, notes, tones, melodies, harmonies of the human voice and of musical instruments. There is even a great difference between the disciplined, and therefore differentiated ear of a cultured person, and the undisciplined, and therefore less differentiated ear of a boor. Similar specializations of structure and function pertain to the other senses; but we may pass them.
The digestive, circulatory, and respiratory systems, and all the other systems of the animal structure, evince the same law.
The lowest form of the circulating fluid, as in sponges, is simply water containing gases and organic particles; and this can scarcely be spoken of as circulating, for it is merely drawn in and then expelled. A little higher in the scale naturalists find a 'chylaqueous fluid,' which oscillates in the general cavity of the sack-like animal. The true blood is another step in development; and even this organized fluid changes its character as the scale advances. Most animals have no heart; and when the organ does first appear, it is but a simple, rudimentary structure, very unlike the complex machine which plays at the centre of circulation in the higher types.
Though fishes breathe through their gills, receiving all the oxygen they require from the small amount of air in the water, the swimming bladder is in them the rudimentary lung—a very simple structure, indeed, when compared with the more complex arrangement for respiration in the higher animals.
Some animals of gelatinous, and therefore flexible structure, perform digestion by folding their bodies over the food, and pressing the nutritious matter out of it: they extemporize a stomach for the occasion. And even in some of their higher types, in such as have a permanent mouth and stomach, the digestive process is simply a squeezing out of the elements of nutrition. The digestive apparatus, from being a simple sack in the polype and similar organisms, becomes, by a continuous unfolding, the complicated structure which we find in the higher animals, with various organs effecting various parts of the digestive change, and even different parts of the same organ having specialized functions to perform.